Some thoughts
If we are using that, there are a few tweaks I'd reccomend.
First, and most obviously, defense bonuses scale down to BAB bonus style progression (fast gets 1/1, smart and charismatic get 1/2, everything else gets 3/4). If we keep the normal defense bonuses, defense will far outstrip offense and combat gets significantly less dangerous.
Secondly, some small measure of level-based soak might be appropriate. If you take D&D and port in straight into the new grim-n-gritty, the overall toughness of each class is maintained: fighters have heavy armor and high BAB, so they're tough; clerics have heavy armor and medium BAB, so they're pretty tough, too; wizards have low BAB and no armor, so they go squish. In d20 modern, hit die is a lot more of a balancing factor, since some classes are explicitly "tough." Since heavy armor is rarer, people still won't be behemoths, and there's a reason to take higher HD classes.
For progressions, I'd reccomend something around 4/20 for d10 classes (rising to 8/20 with robust or for d12 classes), 3/20 for d8, and 2/20 for d6. So Jack would have almost reached the point where he's earned a point of soak (1/10 from Cha 1, 2/5 from Tough 2, and ~1/4 from Fast 1 and strong 1) and taking robust would put him over the edge. Or, if you doubled all the values above (d10 is 4/10, d8 is 3/10, d6 is 2/10) to make everything a bit tougher, Jack would have (.8+.6+.2=1.6) 1 point of soak already, and 2 if he takes robust.
Anyway, whatever we do, we should probably get things movin'.
Trailing thought- the new GnG also dumps penetration, which means the simplist route is to simply add a special quality to rifles which says "ignores up to X points of soak from natural armor or armor. Depending on the value of X, the value of rifles skyrockets.
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